The present invention relates to a communication terminal connected to a communication network and, more particularly, to a system for a communication terminal having an interface which conforms to The Comite Consultatif International Telegraphique et Telephonique (CCITT) Recommendations, I. series, to respond to an incoming call.
The CCITT named a network for offering a digital communication service Integrated Service Digital Network (ISDN) and, in 1984, gave Recommendations, I. series (Redbook) on basic ISDN items. In accordance with the Recommendation I. 451 (CCITT Redbook Vol. III, Fascicle III.5/Q.931), on receipt of a call accept (SETUP) message from a network, a terminal compares the values of its own attributes with those of the service attributes of the call which are contained in the SETUP message so as to decide whether or not to respond to the call and, only when it has decided to respond, sends a respond message CONNect (CONN) to the network.
A more specific procedure is as follows.
A message sent from a network to a terminal for connection is called a SETUP message. The SETUP includes various kinds of information elements such as protocol disciminator, call reference, message type, bearer capability, channel identification, progress indicator, terminal capabilities, display, keypad, signal, switchhook, calling party number, calling party subaddress, called party number, called party subaddress, transit network selection, low layer compatibility, high layer compatibility, user-user information (CCITT I.451/Q931). Among such information elements, the bearer capability, low layer compatibility, high layer compatibility and user-user information are used for compatibility checking which the terminal performs for deciding whether it may answer a call that has arrived. These information elements other than the bearer capability are ommited, as the case may be. The terminal has therein a classified table of those contents of the above-stated information elements with which it is compatible, and their items, i.e., attributes as well as their values. The terminal decides whether it may accept an incoming call by comparing the contents of a call message that has arrived with those stored therein.
Referring to FIG. 1, assume that a customer premises 20 which is incorporated in an ISDN network 10 includes a point-multipoint bus 22 which is connected to the network 10 via a network termination (NT) 21, that a plurality of terminal equipment (TE's) 23, 24, . . . , and 2n are present on the bus 22, and that the TE's 23, 24, . . . , and 2n share a single subscriber number and have no subaddress.
As shown in FIG. 2, in the case of a prior art response system as prescribed by the Recommendation I.451, when a SETUP message is broadcast from the network 10 to the TE's 23, 24, . . . , and 2n . If a plurality of TE's, e.g. TE's 23 and 25 decide that they are ready to respond to the call, each of the TE's 23 and 25 returns a CONN message to the network 10. Then, the network 10 sends a CONNect ACKnowledge (CONN ACK) message to that TE 23 which returned the CONN message first, thereby allowing the TE 23 to answer the call but not the TE 25.
Since the ISDN terminal arrangement described above is so designed that, if a plurality of such terminals are connected to the same point-multipoint bus, the call be accepted by the terminal which has returned a CONN message first, the choice of the terminal may not be consistent with the order of priority desired by the ISDN user for the responses of the ISDN terminals to incoming calls.
Meanwhile, there are also proposed a method by which a switching system differentiates the ISDN terminals and sequentially designates a subaddress for each ISDN terminal according to the order of responding priority to properly connect the incoming call, and another by which a special network termination (NT) 21 is installed in the premises and given the same function as the switching system mentioned above, but they still have the disadvantage that the ISDN terminals on the subscriber side cannot set the priority order freely and simply on their own.
This leads to the problem that, if there is one among the communication terminals which is always quicker than the others to return a CONN message, every incoming call will be accepted by this communication terminal, but connection can be made to no other terminal, even if communication is desired with another terminal and call initiation is attempted many times.
In this connection, a response system of a terminal to an incoming call has been proposed in the U.S. Patent Application Ser. No. 043,462 assigned to the same assignee as the present application. This system is provided with a plurality of communication terminals sharing a single subscriber number, to enable each of the communication terminals to complete the line connection for the incoming message with means to store the calling party's number and attributes of the incoming call if the terminal finds itself different from the really desired party and another means to delay the timing of returing a CONN message in response to every subsequent incoming call having the same calling party number and attributes.
The above-mentioned method for priority order control for responses of an ISDN terminal to incoming calls has another disadvantage that, if the ISDN terminal is composite and has a plurality of communication service functions and the constituent unit in ISDN terminals differ from one another in functional capacity, only one priority order is available for the composite terminal even if one priority order is desired for one communication service function (for instance fascimile) and another for another function (for example packet communication).